Fortunately, end-to-end joints are a rarity. Most woodworkers avoid them because the porous ends of boards provide relatively weak gluing surfaces compared with edges or faces of the wood. Sometimes, however, they're unavoidable, or you may want to add extra strength to the end of a board to be inserted into a rabbet or dado.A thicker version of glue size can help. Mix two parts water to one part white or yellow glue until the consistency is uniform.When you reach the glue-up stage, brush or dab a small amount of the mixture onto the end grain to be glued, allowing it to penetrate into the wood. Immediately after, apply glue at regular strength and clamp the parts together until the glue dries.